Hi Nash6655,
Unfortunately you are in a difficult position of having to make an exchange from a chain store that has very little in the way of good quality and value mattresses. Post #66 here and the posts it links to will be well worth reading and will give you some ideas about how you may be able to make the best of a bad situation.
The goal is either to exchange for a mattress that is suitable for your needs and preferences and uses high quality materials in the comfort layers (which are generally the weak link of a mattress) or to exchange for a very firm mattress which uses the least possible amount of lower quality materials in the upuer layers and then add your own comfort layer in the form of a topper on top of this. The first option may not be possible at Sleep Country (and they may not be able or even willing to disclose the quality of the materials in their mattresses) so the odds are good that the second option (very firm mattress with the least possible amount of questionable materials and then adding a topper) may be the best way to go. This way you can control the quality of the materials that are used in the upper layers of your sleeping system and the topper will not only extend the life of any lower quality materials below it but can also be replaced if it softens or degrades faster than the rest of the mattress without having to replace the entire mattress.
Phoenix